Ongoing refinement of pollution-control strategies is not going to go away any time soon. As far as diesel engines are concerned—be they in light- or heavy-duty applications—that fact applies particularly to the continuing suppression of NOx. Cooled exhaust gas recirculation (EGR) plays an important part because of its role in significantly reducing NOx output. With ample experience in the development and production of EGR systems, Kolbenschmidt Pierburg has developed an aluminum module that it says permits both targeted cooling and control of the EGR mass.
In designing the new cooler module, the Pierburg team developed what it describes as a “novel” exhaust gas routing arrangement via specially shaped fins that contribute to long-term cooling performance. The cooler is suitable for both low- and high-pressure processes and can be incorporated in the intake manifold of engines with matching design features.
Made entirely of pressure die-cast aluminum, the exhaust gas cooler transfers the heat to the coolant. Depending on the operating point, Pierburg says it reduces the exhaust gas temperature by over 600°C (1080°F), contributing not only to a reduction in combustion temperature and NOx emissions but also lowering the temperature loads on the downstream engine components.
Until recently, coolers have consisted mainly of welded stainless steel panels, a relatively expensive production method. Another benefit of aluminum is that stainless steel has a thermal conductivity of about 15 W/m·K, while pressure die-cast aluminum at about 150 W/m·K is 10 times more efficient. As a result of aluminum's thermal conductivity, high cooling performance can be achieved with a relatively small footprint.
Pierburg says the patented design of the discontinuous fin structure is beneficial because it encourages the turbulent mixing of the exhaust gas flow. The fin structure counteracts direct fouling; encourages convective heat exchange; and permits the exchange of substances, pressure, and heat transverse to the mainstream direction. Those features contribute to a self-cleaning effect of the fin structure. It takes advantage of Kelvin-Helmholtz instabilities, which are exploited when a swirl occurs at the shear layer of two parallel-flow layers with varying flow velocities. An example of this is the trail of vortices visible downstream from a rock in a river.
For the engine to reach its operating temperature as quickly as possible, the exhaust gas cooler has a bypass valve that is actuated either electrically or pneumatically. The exhaust gases are routed through this valve and the bypass channel until the required engine operating temperature is reached.
With the engine at its operating temperature, the bypass is closed so that the cooler can perform its actual task and reduce the temperature of the recirculated exhaust gases. The effective lowering of the combustion temperature achieved through this process, combined with corresponding oxygen abatement, contribute to reduced NOx.
Another advantage cited by Pierburg of the new EGR cooler is its integrated assembly. Its individual components—the EGR cooler with its coolant flow/ bypass valve alternating on-demand between cooler and bypass operation as well as the EGR valve for controlling the EGR rate—have been united in a single EGR module.
Aluminum was the enabler in combining the EGR module and intake manifold in a single component, resulting in an integrated intake manifold module with an EGR and cooling system.
By fine-tuning a new welding method developed for die-cast aluminum parts, it became possible to weld all the internal interfaces of the module together rather than use the type of temperature-resistant seals and supplementary fasteners typically employed.
In addition to meeting high cooling performance, minimum cold fouling, and good resistance to elevated exhaust gas temperatures, the die-cast aluminum construction EGR cooler provides a weight advantage over older designs. Also, because of the corrosion properties of aluminum, all requirements in terms of durability are inherently met.
The aluminum cooler is currently in production in light-duty applications, with heavy-duty applications in preparation for the near future.